Convert to yarn workspaces

[nickofthyme]

Is your feature request related to a problem? Please describe.
Switch to use yarn workspaces.

This would enable to following:

• Cleaner dependencies in bundled library
• Separate dependencies for app and library
• Covert react, etc. to a peerDependency which will improve bundle size and usability.
• Remove the dependency on node@>10 package.json "engine" is too restrictive #359
• All your project dependencies will be installed together, giving Yarn more latitude to better optimize them

Tasks

• Refactor src into packages/elastic-charts/src - build(workspaces): implement src as workspace package #1178
• Refactor src/utils/d3-delaunay into packages/d3-delaunay
• Refactor/cleanup src/mocks into respective packages/ or top-level utils. Might consider using a packages/mock-data to store all sample charts mock datasets.
• Refactor playground into apps/playground
• Refactor storybook and stories into apps/storybook
• Refactor storybook-docs into apps/storybook-docs